Every item in the store is taged with a small mag RFID tag. Instead of a conventional chackstand you place your bags on a conveyor that transports them into a high power RFID scanner similar in structure to an airport baggage xray. This device weighs the bags and scans the item tags totaling the purchase.
You pass through a portal that scans you for tags then pay for the items on the other side. Once you have paid the scanner disgorges the bags and you are on your way.